The Rho family of small GTPase proteins are key regulators of many cellular processes including cell shape, cell motility and invasion, cell-cell interactions, cell proliferation, differentiation, cell cycle progression, gene transcription, apoptosis, and wound repair. Inappropriate activity of these proteins has been described in a number of human diseases such as hypertension, inflammation, and cancers. The long-term goal of this project is to understand the mechanisms of Rho GTPase action required for normal cell behaviors and the developmental consequences of these processes going awry. Rho GTPases are required during early Drosophila development where they are essential in orchestrating the cell shape changes and coordinated cell movements of morphogenesis. Loss-of-function mutations in the Drosophila Rho1 gene exhibit bothmaternal and zygotic defects in several morphogenetic movements. Zygotically, dRho1 shows defects during gastrulation in dorsal closure and head involution. Two phenotypes are associated with reduction of maternal dRhol1: the actin cytoskeleton is disrupted in egg chambers, especially in the outer ring canals and embryos, display patterning defects arising from the failure to maintain proper segmentation gene expression. This study proposes to investigate the mechanisms of dRho1 function in Drosophila. The specific aims of this proposal are to determine the regulatory mechanisms underlying specific dRho1 developmental phenotypes by determining the developmental and physiological outcomes of perturbing specific Rho functions, and to perform genetic and biochemical analyses of two specific dRho1 pathways mediated through its interaction with Cappuccino, a formin homology protein, or alpha- and p120- catenin, adherens junction components. Having a mutation in dRho1 allows us to complement existing biochemical and cell culture studies by analyzing the phenotypic consequences of dRho1 loss-of-function in a developmental and organismal context, to separate the effects of dRho1 from cross-talk with other Rho family members, and to employ the powerful genetics of Drosophila to help unravel the complex nature of their biological properties. Our results are expected to have wide implications as Rho GTPases and the diverse cellular mechanisms they mediate are conserved across a wide range of organisms.